1. Technical Field
The present invention relates to cladding pumping fiber lasers, and in particular relates to the fiber cladding geometry structure and method for construction of high-efficiency fiber lasers for various applications including optical amplifiers.
2. Background Art
The development of diode pumped fiber lasers has been rather successful recently. The scaling of various physical effects has greatly benefited this development. Diode lasers can provide concentrated pumping energy and thus enhance the efficiency of fiber lasers. The long thin geometry of fiber also makes heat removal much easier than in bulk solid state lasers. In end-pumped fiber lasers, a large outer cladding is used in cladding pumping. Pump light, often piped through fibers from pump lasers, enters the outer core, where it is confined so that it passes through the inner core, exciting the laser species. Stimulated emission from the laser species remains in the inner core. By converting the low brightness beam from the pump diode bar into a tighter beam, pumping a fiber laser can multiply brightness by a factor of more than 1000. By using such configuration, Polaroid, for example, reported a diode cladding pumping fiber laser reaching 35 W (M. Muedel et al CLEO ""97, Postdeadline Paper CPD30, Baltimore, Md., 1997).
Currently, a typical fiber laser device includes a tens-of-meters double clad silica fiber with a small diameter and small NA core doped with active species, centered within a much larger inner cladding, surrounded by a soft low index fluoropolymer providing an acceptance NA of 0.45 for pump radiation. Pumping laser beams from laser diodes are coupled into the fiber inner cladding through the dichroic end mirror. (HR laser, HT pump). Among other things, the cladding pumping geometry and the coupling efficiency of pumping laser diode array are the main factors under intense research. Proper geometry is essential for increasing the efficiency of cladding pumping. Good method of coupling will allow more power to be injected into the fiber laser, which in turn also increase the efficiency of a fiber laser.
There are many patents dealing with cladding pumping for various applications including optical amplifiers. U.S. Pat. Nos. 5,533,163, 4,829,529, 4,815,079, 6,157,763 disclose various cladding cross-sectional geometric shapes such as circle, rectangle, convex polygon (triangle, rhombus, hexagon), or modified circles. These cladding boundary (CB) shapes, however, have certain obvious disadvantages. The main disadvantage is the presence of local modes. During our research, we discovered that the presence of local modes is one of the main factors decreasing the efficiency of a double cladding fiber. Essentially, the pumping beam localized in such modes in the inner cladding can not enter the core or can not enter the core efficiently.
When skew ray is reflected on an arbitrary cylindrical surface, the projections of incident ray and reflected ray on the principal cross section are like the light ray reflected in this plane. Therefore, we can use the behavior of light beams in the principal cross section of cylindrical surface to determine local modes of fiber cladding.
The boundary shapes of efficient fiber cladding geometry include circular, rectangle, right triangle, isosceles triangle, and rhombus. As a comparison with the current invention, the local modes in fiber cladding with different boundary shapes are summarized as follows.
FIG. 1 shows a schematic illustration of a light beam path in prior art circular fiber cladding with a circular cladding boundary CB. Because the sag of ray keeps constant in multiple reflection, the light beams LB0 initial at the outer region can not reach central region through multiple reflection. Therefore, the center position is not a good location for core A, and the core must be close to the boundary as position B although center position is usually more preferred due to the structures of fiber connections. FIG. 2 is a schematic illustration showing the local modes in prior art rectangular fiber cladding with a boundary CB. There are two types of local modes in the rectangular cladding. One is the light beam perpendicular to the boundary (LB1, and LB2), the other one is the light beam parallel to the line joining two corners (LB3). These light beams form different close cyclic loops in the rectangular boundary. Besides these two fiber cladding geometry shapes, there are also right triangle, isosceles triangle and rhombus. For right triangle case, the light beams perpendicular to the hypotenuse form different close loops in the right triangle boundary as shown in FIG. 3 (LB4 and LB5). In isosceles triangle fiber cladding, there are two types of local modes as shown in FIG. 4. One is the light beam perpendicular to the leg (LB6), the other type is the light beam parallel to the base (LB7). Rhombus cladding behaves like two isosceles triangles. The local modes in rhombus cladding are the same as in isosceles triangle as shown in FIG. 5.
To increase the efficiency of a double cladding fiber, sometimes bending in the fiber structure is suggested to provide perturbation in the modes propagating in the multi-mode cladding. However, the effect of bending on perturbation is not clear, and can not be accurately predicted. Other methods include introduce mirror surfaces into the otherwise circular inner cladding, such those shown in FIGS. 6, 7, and 8. Although some of the local modes can be destroyed with these geometry shapes as the inner cladding, the integral reflection loops still exist due to the presence of geometrical symmetry as shown by the localized beams LB8, LB9 and LB10, respectively.
If a fiber cladding has some local modes and the core is not in the region of the local modes, the pumping light beam of local modes can not reach the core and the pumping efficiency will decline. But the presence of local mode was not noticed in the past and therefore the improved cladding geometry could not destroy all the local modes due to the presence of symmetry. The efficiency of fiber was not maximized. It will be much more favorable to find new cladding geometry structures so that local modes are destabilized or destroyed so that the pumping beam can easily enter the core. In this way, the efficiency can be increased and the length of fiber lasers can be reduced.
Currently, the leading company in fiber laser research and manufacturing is Polaroid. One fiber laser of Polaroid was reported to have high efficiency (about 65%), but this efficiency is the ratio of pumping laser power entered the optical fiber and the output power of the fiber laser. Therefore, the efficiency of coupling or power injection is not considered. In this Polaroid fiber laser, three fiber-coupled SDL P6 diodes are spatially combined and de-magnified into a rectangular cladding. The slop efficiency of the diode lasers is only 0.5 W/A, while the efficiency of a non-fiber coupled diode laser is much higher, 1.25 W/A. Therefore the total electric efficiency of this Polaroid fiber laser is not high. In addition, due to the limitation of the cladding geometry, tens of meters of fiber must be used in this state-of-the-art system. Furthermore, since it is difficult to couple more beams into an optical fiber with the coupling method used in the Polaroid systems, it is difficult to develop a fiber laser with even higher power, such as 1000 W CW. It is therefore necessary to find new methods to couple high power into optical fibers and improve the pumping efficiency.
Accordingly, it is the principal object of the present invention to provide a high-efficiency cladding pumping fiber laser for various applications including optical amplifiers by using fibers with efficient cladding geometry.
It is also an object of the present invention to provide a method and apparatus with which an efficient double cladding laser fiber can be made that has a cladding geometry that can avoid or minimize local modes.
It is another object of the present invention to provide methods of making double cladding fibers with inner cladding having shapes that destabilize and destroy local modes of light beams within inner cladding.
It is another object of the present invention to provide novel cladding geometry so that the length of laser fibers can be reduced.
To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, a preferred article for gain applications may comprise a diode laser pumping source comprising at least one laser diode array, a double cladding laser fiber having an inner cladding and an outer cladding with the core of the fiber doped with active species, and a coupling optical system that is disposed between said diode laser pumping source and the aperture of said fiber, and focuses the beam from the diode laser pumping source into the inner cladding of the double cladding laser fiber, wherein the cross-sectional shape of said inner cladding is an asymmetric and symmetry-broken polygon that destabilizes local modes of light beams within said inner cladding. One of more of the boundaries of the polygon shaped cross-section of the inner cladding can also be arc. In the fibers of this invention, the cross-sectional shape of said inner cladding can also be a multiple-imaging cladding or a rectangular-like multiple-imaging cladding. The article or apparatus for gain applications summarized above can be a fiber laser by further including reflector means at each end of the fiber, or it can also be an optical amplifier by further including a coupler means so that input signal can be coupled into the double cladding laser fiber and amplified.
Additional objects, new features and advantages of the present invention will be set forth in part in the following description. Further scope of applicability of the present invention will become apparent from the detail description of the invention provided hereinafter. It should be understood, however, that the detailed description of the invention and the specific examples presented, while indicating preferred embodiment of present invention, are provided for illustration purposes only, because various changes and modifications within the scope and spirit of the present invention will become apparent to those of ordinary skill in the art from the detail description of the invention that follows.